Finished Pieces

Looping/re-triggering with transistors

    The drawing above shows one of the many ways you can utilize transistors in circuit bending.
    The transistor in this drawing is there to automatically trigger the circuit to make sound. The circuit shown is a generic sound board from a generic kids toy.

    Let’s go over the basics. The black circle is the brain where the real work of the circuit is done and the sound is recorded or generated. To the right of that is a pitch resistor above is a transistor which acts as an amplifier for the speaker. next to that are two button contacts. when these are connected it triggers whatever the circuit is meant to do.

    I’ve got a situation where I want the circuit to continuously make sound. In some cases you can simply wire a jumper across the button contacts and it will keep triggering sounds. In MANY cases the brain wants to “see” a new button press in order to trigger a sound. Want I want in most cases is for the sound to trigger immediately after the sound has finished, and that’s what the transistor does.
    One important factor in this circuit is that everything we want to play with is tied to the supply voltage, meaning the button contacts are connected to the trigger in(on the brain) and +V, the speaker is amp(transistor) out and +V. This is important to note when using transistors because the transistors will basically allow you to control the current which goes into or out of these points (trigger, pitch, audio). Knowing what the brain wants to “see” will tell you what kind of transistor configuration to use.
    In this case the brain wants to see positive voltage at the trigger point.

    The emitter of the transistor is connected to the trigger pin and the collector is connected to the supply voltage (4.5V). When the BASE pin of the transistor is connected to ground or negative voltage, current will flow between the emitter and collector, which will trigger the circuit to make sound.
    The BASE pin in this circuit is connected to the audio out and is then pulled low (to ground) through a 100k resistor. That means when there is no signal at the base it will be pulled low (current will flow between collector and emitter). When there is a signal (ie. sound is being generated) there will be NO flow between collector and emitter. So as soon as it stops making sound, the lack of sound activates the transistor which triggers another sound.


    Sorry, no sound files for this piece.

    11 Responses to “Looping/re-triggering with transistors”

    • 1

      Ian Michael Ferguson said:
      December 4th, 2011 at 12:35am #

      Nice. Will this solve our Saw sampler playback length issue?

    • 2

      casper said:
      December 5th, 2011 at 11:26pm #

      Hey! Funny you should ask. I excitedly tried this mod on the saw sampler and was disappointed to find that it doesn’t work. BUT I was determined to find a solution and fired up my scope to find a signal I could use. I don’t know how I missed it in the past but there is a pin right above the pitch pin that goes high during playback and then switches low. This can be used to trigger your playback and results in perfect looping. Just connect the center pin of your re-triggering transistor to this pin and your ready to loop.

    • 3

      Ian Michael Ferguson said:
      December 6th, 2011 at 2:11am #

      Hooray! I’ll give that a shot. I’m planning on putting one into a guitar pedal with a blend mix knob and other craziness. I have high hopes for it. You think a 3v 500mA voltage regulator will work to take pedal 9v down to something Mr. Saw will tolerate?

    • 4

      dIsLoCaTiOnS said:
      January 30th, 2012 at 12:02pm #

      Just tried this out on a few toys…great stuff and easy as hell!!! One question. Would the rate be altered by substituting either resistor with a pot?

    • 5

      casper said:
      February 16th, 2012 at 8:19pm #

      No sir. Those resistors are just there to make the transistor work as a switch. They have nothing to do with the pitch of the circuit they are triggering.

    • 6

      Pigsnoot said:
      March 2nd, 2012 at 8:40pm #

      Maybe a cap will influence the trigger rate?

    • 7

      Pigsnoot said:
      March 2nd, 2012 at 8:41pm #

      Oh and by the way: Casper you’re amazing!

    • 8

      Joel Saunders said:
      July 3rd, 2013 at 5:09pm #

      What about toys with multiple sounds/buttons? I’ve a toy phone this wold work great in.

    • 9

      casper said:
      August 1st, 2013 at 1:55pm #

      @ Joel. That should work fine!

    • 10

      Michael said:
      November 1st, 2013 at 11:04am #

      Hey Casper,

      I have a few questions(since I’m new to this stuff).

      Where could I buy these components?
      How small can they get(planning on making a replica pistol that makes noise)?
      How fast can the sound be repeated(like for a assult rifle, maybe 6 times per second)
      Could I also make the sound repeat 3 times quickly for every trigger/button press?(Different weapon, different firing mode)
      [trying to make them as accurate as possible, for Costumes]
      How hard is it to get/record sounds for these?
      Is it possible to make it stop the sound after a few seconds(10 seconds maybe)?

      Sorry for the random questions.

      Thanks, Michael.

    • 11

      Wayne said:
      February 16th, 2015 at 10:33am #

      Hey Casper,
      very cool website with a lot of usefull informations !!!

      I just found the comments here where are you speaking of the saw looper…i modded it like suggested at the getlofi website with a 2n2222 transistor…the loop works,but very poor…it just loops a very small part of the sample…will your schematic fix this problem ?
      Any help is much appreciated !!!


    Leave a Reply

    If you're not already logged in, there might be a question you need to answer before you post to prove you're human. It's there to help stop comment spam.